1. Field of the Invention
This invention relates to a compact quench box design for multi-bed, mixed-phase, cocurrent downflow fixed-bed reactor.
2. General Background
Reactors used for hydroprocessing of petroleum streams often require multi-bed design. Multi-bed mixed-phase cocurrent downflow fixed bed reactors are commonly used in the refining industry for hydrodesulfurization, hydrodenitrogenation, hydrodewaxing, aromatic saturation, hydrocracking, and hydroisomerization dewaxing reactions. Similarly, exothermic reactions, such as two-phase exothermic chemical reactions, in chemical plant reactors may also require a multi-bed design.
In such multi-bed reactors, a catalyst is provided in a series of two or more vertically spaced beds. The material being treated is introduced at the top of the reactor so it flows downwardly through the beds during treatment. Reaction components may be added at the top of the reactor with the material to be treated and additional reaction components may be added between beds.
As the material being treated moves downwardly from bed to bed, it may be desirable to redistribute liquid components over a bed for more even treatment in each bed, and/or, with exothermic reactions, to cool the reactants between each bed. Quench boxes are commonly used between beds for these purposes. The requirement for a quench box may be driven either by the need for liquid redistribution for improving the contact efficiency between reactants and catalyst or for improving temperature control in the reactor due to the exothermic reactions in the catalytic bed.
Redistribution of Liquid: The desire to design hydroprocessing reactors at reduced pressure drop, which saves capital investment by reducing the size of the recycle compressor, allows the liquid to trickle over the catalyst surface over the cross sectional area of the reactor by gravitational flow. Consequently, liquid tends to flow gradually in non-homogeneous fashion at about ten to fifteen feet down from the distributor. Chou in I&EC Process Design & Development vol, 23, p. 501 (1984) stated that “Since liquid trickles over the catalyst's external surface under gravity, liquid will eventually reach its equilibrium distribution between the wall liquid and the liquid trickling over the catalyst surface. Consequently, a uniform liquid distribution attained near the top of the packed bed does not guarantee a uniform liquid distribution a few feet down the bed.” Non-uniform flow of liquid in a catalytic bed affects the efficiency of the catalyst utilization. Therefore, it is common practice in the industry to limit the bed height and to provide flow redistribution quench boxes at a pre-determined maximum bed height for improving catalyst utilization.
Temperature Control: Hydroprocessing reactors or any chemical plant reactors involving highly exothermic reactions, (e.g., olefin saturation, aromatic saturation, hydrodesulfurization, hydrocracking, or hydrodenitrogenation reactions) may require multiple beds for the temperature control, for product quality control, (e.g., color body for diesel product), or for extending the catalyst cycle life. Generally, a stream at a lower temperature, (i.e., quench gas or quench liquid) is used to adjust the temperature in the quench box. Intimate mixing between the quench stream and the hot vapor-liquid reaction two-phase fluid stream (hereinafter, “hot reaction two-phase fluid stream”) coming from the bed above at a higher temperature is critical for providing an improved performance in the catalytic bed located below the quench box.
The commonly used quench gas is normally supplied from the discharge side of the recycle compressor located in the process unit. Quench gas injection into the quench box serves several important roles. The quench gas is used to adjust the reactor temperature at the optimal conditions. The quench gas, if hydrogen, supplements the hydrogen depleted from the exothermic reactions in the bed above the quench box. Quench gas addition in the quench box also ensures proper vapor/liquid phase partition in the bed below the quench box to ensure optimal operating conditions.
The United States Environmental Protection Agency (EPA), in announcing its new sulfur content rule of Dec. 21, 2000, stated that “To ensure cleaner-running trucks and buses, today's action also requires that sulfur in diesel fuel be reduced by 97 percent.” This new EPA rule sets the sulfur standard in the diesel fuel for on-road vehicles at 15 ppm effective June 2006 as opposed to the present standard of 500 ppm sulfur in diesel fuel. This EPA regulation for the diesel fuel sulfur content provides impetus for the refining industry to search for means to satisfy the requirement of reduced sulfur in diesel fuel while minimizing capital investment. Process revamp (i.e., for a process unit having an existing reactor) to increase the catalyst volume by adding a new multi-bed fixed bed reactor, coupled with the loading of the most advanced high activity catalysts, seems to satisfy the requirement with minimal cost. Consequently, the compact quench box design for the multi-bed, mixed-phase cocurrent downflow fixed-bed reactor allows the refining industry to satisfy this new EPA rule with reduced capital investment for a process revamp or grassroots design (i.e., a new process unit having a new reactor design).
Although voluminous disclosure of quench box design is available in the patent literature, there are still needs to developed a compact quench box design to promote the required intimate mixing between the quench stream and the hot reaction two-phase fluid stream while reducing capital investment or unit down time during turnaround (i.e., catalyst changeout). The savings on reactor height can be used to load an incremental catalyst volume to improve the performance of the reactor or to reduce the total weight or the capital investment of the reactor.
3. Discussion of the Prior Art
Hanson, et al., U.S. Pat. No. 3,541,000, discloses the use of two trays below the mixing chamber for effecting the distribution of vapor and liquid onto the bed below the quench box. Quench gas is introduced below the mixing chamber which reduces the extent of thermal equilibrium achievable for contacting the quench gas with the hot reaction two-phase fluid stream at a low vapor-liquid interfacial surface area. The void space (hereinafter, “voids”) between the catalyst support beams is not utilized for quench gas and the hot vapor-liquid reaction two-phase fluid stream contact.
Smith, et al., U.S. Pat. No. 3,824,081, discloses a quench box with mixing device followed by a distributor assembly consisting of one perforated tray and one distributor tray with v-notch downcomers to provide the uniformity of fluid flow onto the bed below. The disclosed design requires excessive reactor height to achieve the required mixing and redistribution. No attempt was made to reduce the quench box height. The redistribution assembly requires two trays to accomplish the desired redistribution of liquid.
Alcock, et al., U.S. Pat. No. 3,977,834, discloses the significance of utilizing the voids between the catalyst support beams for a compact quench box design. While the effective use of the voids between the catalyst support beams for quench gas distribution is novel, the quench gas distribution device disclosed in the patent utilizes a grid to introduce quench gas through perforated pipes. This is an ineffective means for achieving thermal equilibrium between the lower temperature quench stream and the hot reaction two-phase fluid stream when compared to controlled injection of the quench gas. The design according to the disclosed patent does not provide an effective sweeping action of the quench gas onto the hot reaction two-phase fluid stream at a higher temperature. A plurality (from two to ten) of “quench boxes” are used in the disclosed patent, but which will not provide uniformity of temperature distribution from different “quench boxes”.
Frohnert, et al., U.S. Pat. No. 4,792,229, discloses the use of a conical collecting plate and a static mixer to achieve desirable mixing. Commercial installation using a design similar to the disclosed patent shows that the use of static mixer requires excessive quench box height to attain desirable mixing. Use of the conical collecting plate also requires greater reactor height than use of a flat collection tray design. Overall, the quench box of the disclosed design requires much greater reactor height than any conventional quench box design equipped with a flat collection tray.
Aly, et al., U.S. Pat. No. 4,836,989, discloses a quench box design similar to that of Hanson in view of the quench box features. However, the disclosed patent has revised the distributor assembly and the quench gas distributor location from those of Hanson. The distributor assembly consists of two trays, a flash pan tray and a final distributor tray. The flash pan tray has perforated plates with liquid passageways and downcomers for vapor passageways. The final distributor has downcomer pipes for effecting the uniformity of fluid distribution onto the bed below the quench box. The quench gas is distributed above the mixing chamber device. The disclosed quench gas distributor requires a bend upward (or could be downward) for the quench gas pipe at the centerline of the reactor. This requires additional reactor height to accommodate the quench gas distributor assembly. The disclosed design does not provide for a compact quench box design in view of the requirement for the bend of the quench pipe at the centerline of the reactor and the two-tray assembly for the distributor system.
Nelson, et al., U.S. Pat. No. 5,989,502, also discloses a quench box design similar to that of Hanson in view of the quench box features. The mixing chamber outlet weir is called an “orifice” in this patent, indicating the intent for the mixing chamber outlet to provide further two-phase interactions. Since vapor and liquid tend to segregate in the mixing chamber, the intended orifice weir outlet design serves little function to promote the interactions between the vapor and liquid phases. This design and the prior art disclosed in Hanson and Aly create an orifice weir outlet for the mixing chamber; this requires an additional manway on the bottom pan of the mixing chamber. The unit turnaround time is extended resulting from the orifice weir outlet design using the disclosed patent and other prior art design.
Grott, et al., U.S. Pat. No. 5,837,208 discloses the quench box design similar to that of Aly and Hanson. The main feature of the design as claimed in the patent is a compact quench box design by providing a rough liquid distributor in additional to the final distributor. In view of the accomplishment with the improved design for the rough liquid distributor, the total saving on the quench box height is still quite limited. Underneath the mixing chamber, the disclosed patent design still requires two distributor trays to provide the flow distribution unto the catalytic bed below the quench box.
Boyd, et al., U.S. Pat. No. 5,935,413 discloses a modified version of Aly and Hanson. Separate mixing chambers are designed for the vapor and liquid phases. After segregation for the two phases and vigorous mixing for the separated phases, the two streams are mixed in the form of thin sheet of liquid impinged by the cooled vapor stream (quench gas is in contact with the vapor phase only in the vapor phase mixing chamber). The drawback of the design based on this patent is its limited thermal equilibrium achievable. The thin sheet of liquid at the outlet of the liquid phase mixing chamber provides very limited interfacial surface area for the heat transfer between the hot liquid and cooled vapor phases. Therefore, the design based on this patent disclosure may not achieve the desirable thermal equilibrium. Two trays underneath the mixing chambers are also required for the fluid redistribution onto the catalytic bed below the quench box.
Rossetti, et al., U.S. Pat. No. 5,152,967, discloses the use of a quench gas pipe that terminates in a centrally located ring sparger that has a plurality of openings. The quench gas distributor is located between the mixing device and the catalyst support mechanisms. The effectiveness of the quench gas to sweep the hot vapor-liquid reaction two-phase fluid stream is limited with this design due to its non-optimal sweeping velocity as a result of the inherent nature of the ring sparger design. The quench mixing device and the distributor assembly in the disclosed design consist of four trays, which require excessive reactor height to accommodate.
The design for the final distributor can take various forms to provide uniform flow of liquid over the catalytic bed below the quench box. Grosboll, et al., U.S. Pat. No. 4,126,540, discloses a downcomer with a side orifice design for the final distributor. Ballard, et al., U.S. Pat. No. 3,218,249, discloses the use of bubble-cap for the final distributor. Jensen, U.S. Pat. No. 4,140,625, discloses the use of venturi-shaped eductor for distributing liquid over the catalytic bed. Effron, et al., U.S. Pat. No. 3,524,731, discloses the use of truncated triangular cutout for the top slots on the downcomer to provide a uniform flow of liquid with varying rate for unit throughput. Smith, et al., U.S. Pat. No. 3,824,081, discloses the use of v-notch downcomer for the final distributor design for achieving uniform flow of liquid onto the catalytic bed. Scott, U.S. Pat. No. 4,235,847, discloses the design of the final distributor for effecting froth flow conditions on the distributor tray. Koros, et al., U.S. Pat. No. 5,403,561, discloses the use of a spray generating device for producing a conical downward spray of mixed phase onto the catalyst bed. The aforementioned types of final distributor designs can be incorporated in this quench box design without sacrificing the significance for a compact quench box design.